Russeting is an important surface disorder in smooth-skinned cultivars of apple (Malus × domestica Borkh.) and pear that results in significant economic losses. The brownish, dull appearance of russeted fruit is unattractive to the consumer who prefers smooth-skinned fruit. In russeting, the primary skin, comprising cuticle, epidermal, and hypodermal cell layers, is replaced by secondary dermal tissue called periderm. This periderm forms in the hypodermis (Meyer, 1944). The periderm consists of meristematic phellogen that produces phelloderm cells toward the inner and phellem cells toward the outer side by cell division (Esau, 1969). Suberin deposition on the cell walls of the phellem is responsible for the brownish appearance of russeted peel.
Microscopic fractures (“microcracks”) in the fruit surface are considered to be the first visible symptom in russeting (Faust and Shear, 1972a, 1972b). Factors causing microcracking often stimulate russeting. Such factors include high humidity, prolonged surface wetness, exposure to freezing temperatures, mechanical injury, and colonization with certain microorganisms (Faust and Shear, 1972a; Gildemacher et al., 2006; Knoche and Grimm, 2008; Simons and Chu, 1978). Mechanical growth stresses of the expanding surface provide the driving force for microcracking (Curry, 2009; Skene, 1980, 1982). Supporting evidence for this relationship comes from the observation that mechanical growth stresses are at maximum during early fruit development when fruits are particularly sensitive to russeting (Knoche et al., 2011; Wertheim, 1982). From this we hypothesize that the incidence of russeting will be higher on surfaces subjected to high relative growth rates and vice versa. Pear fruit is a particularly suitable crop to test this hypothesis because it offers contrasting surface growth rates and relative growth rates in the surface area within the same fruit, thereby normalizing for fruit-to-fruit variability.
The objective of our study, therefore, was to test this hypothesis using two cultivars of pear, which differ in susceptibility to russeting; ‘Conference’ is classified as being highly susceptible, whereas ‘Condo’ is intermediate susceptible. The fruit was partitioned into three regions of contrasting shape and a geometrical model was developed to quantify growth rates of the fruit surface. These were related to the development and final incidence of russeting in those regions during fruit growth and at maturity.
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